Conventionally, an AC distribution system based on a commercial power has been used to supply power to various electric devices in a house. Recently, there becomes widely used a DC decentralized power supply system using a solar cell (photovoltaic power generation), a fuel cell, a storage battery, or the like provided in the house. Also, in order to reduce a power loss in converting an AC power into a DC power in each electronic device, there is proposed an introduction of a home DC distribution system. In these cases, a DC distribution system is required to be installed in addition to the related conventional AC distribution system.
In case of the AC distribution system, since a current zero cross point exists once every half period, current can be easily cut even when a contact switching type switch is used. However, in case of the DC distribution system, since there is no current zero cross point and an arc is generated in cutting current even at a relatively low voltage (e.g., about 40V), the distance between contacts needs to be increased or an arc extinguishing function such as an electromagnet or the like needs to be provided. This makes, compared with the switch in the AC distribution system, the switch itself in the DC distribution system larger.
Further, in case of photovoltaic power generation, a fuel cell or the like, since it includes a high voltage of a few hundred V, it is necessary to perform a powerful arc extinguishing function as well as to increase the distance between contacts in order to effectively cut a current flowing therethrough. Thus, it is not easy to substitute a switch for the existing AC distribution system with a switch for a DC distribution system or combine the switch for the DC distribution system with the switch for the existing AC distribution system. Furthermore, the presence of switches each having a different size degrades aesthetical appearance and is not desirable in terms of interior design.
There is proposed a DC switch that does not generate an arc by using a MOSFET as shown in FIG. 16 (see, e.g., Japanese Patent Application Publication No. 2005-293317). In the patent, a source of a MOSFET Q1 needs to be connected to a negative electrode of a DC power source, and current can be blocked only at the negative electrode side. However, when a personal protection against an electrical shock or the like is taken into consideration, it is preferred to block the current at a positive electrode side.
Further, when a polarity is erroneously connected between the DC power and the switch, the current flows by way of a diode included in the MOSFET unit, and, accordingly, the MOSFET does not serve as a switch. Furthermore, when the DC power is supplied from a storage battery and the DC switch is used for charging and discharging, a direction in which the current flows is inverted in charging and discharging while supplying a DC voltage. In this case, the MOSFET operates only in either one of the directions of the currents, which makes function as a switch incomplete. In other words, the DC switch cannot be employed in the AC power system in which the flow direction of current is inverted every half period.